Background--Clinical Need
The hazard of air embolization related to either cardiotomy or pump oxygenator accident is well recognized by all cardiac surgeons. Air embolism can cause devastating injury in the distribution of both the coronary and cerebral circulations. In the experimental animal, air in quantities of 0.5 cc/kg injected into the left side of the heart can cause death from coronary embolization. When air is injected into the carotid artery of the dog, 0.25 ml/kg causes marked neurologic damage.
It is generally believed that the most frequent source of air embolization is air trapped in the left heart chambers related to cardiotomy, and research efforts are still being directed in this area, since this hazard is not yet completely solved. Neurologic sequelae after coronary artery operations without cardiotomy, although less common, continue to be seen occasionally and may be related to air or particulate emboli in the ascending aorta.
The hazard of air introduction during aortic cannulation, related to manual squeezing of the clamped air-containing cannula during its introduction has been pointed out by researchers. In this application a small device, such as the radiometric antenna, that could be clamped onto the cannula configuration might be useful in detecting small emboli.
Other applications of an air emboli detection device include angiographic examinations. These procedures have become routine in even small hospitals throughout the country. The introduction of air into the vascular system, especially during intracerebral examinations, is an occurrence which, expectedly, has great morbidity and mortality. The potential for inadvertent introduction of air is present each time an angiographic study is performed. The mechanism of damage due to air emboli is not well understood, but the devastating effect, especially in the cerebral and coronary circulations, is well known. The clinical problem which has precipitated an intracerebral angiographic examination may be mimicked by the introduction of air into the cerebral vessels. Therefore, the occurrence of air emboli may not even be suspected.
Currently, the only method to insure that no air is introduced into blood vessels during the angiographic examination is the adherence to meticulous angiographic technique. Visual detection of air bubbles in the system and their removal prior to injection of contrast agents is the only current method available. For obvious reasons no one has done a controlled study on humans to see what quantity of air introduced into carotid or vertebral arteries would be necessary to result in clinical signs. Even small amounts of air however would be expected to have the potential for clinical sequelae. Canine studies suggest that air emboli of 1 ml injected into a pulmonary vein result in death from coronary air embolism. Arterial air emboli of approximately 0.5 ml in humans are of sufficient size to cause fatal arrhythmias from focal coronary ischemia. One might reasonably expect the critical lethal air embolus volume in the cerebral circulation to be even less than 0.5 ml. As previously mentioned, it is difficult, if not impossible, to state with certainty the critical volume of air emboli which result in morbidity. Given that fatal air emboli of 0.5 ml or greater should routinely be detected visually by careful techniques, a system to detect air emboli would need to be sensitive to bubbles of 0.1 ml or smaller to routinely detect the smaller bubbles which have greater potential to be "missed" by careful techniques.
An effective air embolus detection device would have application to all pump oxygenator and contrast injection systems currently in use throughout the health care delivery system. In order for an air emboli detector to be of value clinically, it must be designed such that it does not prolong the existing procedure (for example, complications in angiography are directly related to the amount of time that the catheter is actually within the vascular system). Ideally, the device should be able to be used without necessitating additional steps in the usual manipulation of catheter, stopcock and pump. To be able to have the detecting device somehow integrated into an existing part of the system would be ideal, as would having the device be sterilizable and/or disposable. The detection device must be "in line" throughout the procedures.